At the well outlet, natural gas is mainly made up of methane, ethane, propane and butane, and it also contains smaller amounts of heavier hydrocarbons. Natural gas furthermore contains various acid compounds, generally carbon dioxide (CO2), hydrogen sulfide (H2S), as well as carbonyl sulfide (COS) and mercaptans (RSH). The mercaptans mainly consist of methylmercaptan (CH3SH), ethylmercaptan (C2H5SH), propylmercaptans (C3H7SH) and butylmercaptans (C4H9SH), and possibly mercaptans having longer hydrocarbon chains that can reach six carbon atoms.
The natural gas deacidizing operation, which is essentially intended to reduce the CO2 and H2S content of the gas, is for example performed by means of an absorption method, using notably chemical solvents like, for example, alkanolamines such as diethanolamine (DEA) or methyldiethanolamine (MDEA). After this treatment, the gas meets the specifications relative to the CO2 content, typically below 2% by mole, and to the H2S content, typically 4 ppm by mole. Part of the light mercaptans, notably methylmercaptan, is removed during this operation. The heavier mercaptans such as ethyl, propylmercaptans and butylmercaptans, or containing more than four carbon atoms, are not soluble enough in an aqueous solution or acid enough to significantly react with the alkanolamines generally used for deacidizing, and a large part thereof therefore remains in the gas. Most of these acid compound absorption methods have a mercaptan extraction efficiency ranging between 40% and 60%.
Document FR-2,873,711 discloses a method for collecting and removing the mercaptans contained in a natural gas by reaction with olefins.
The present invention is aimed to improve the method described in document FR-2,873,711.